The BHA is to investigate the reasons why Frankie Dettori ditched one of his rides at Newbury last Friday. Photograph: Jed Leicester/Getty Images

The British Horseracing Authority saidon Wednesday that it is still investigating Frankie Dettori's decision to give up a booked ride at Newbury last week, reportedly to show his support for the Horsemen's Group's tariff on prize money.

Dettori gave up the mount on the Robert Mills-trained Diamond Vision, who started third-favourite at 4-1. The jockey, one of the few who can persuade significant numbers of betting-shop punters to back a horse simply because he is riding it, was later quoted as saying that Godolphin, his principal employers, had asked him to do so "because it is under the tariffs".

However, the BHA is continuing to investigate an apparent discrepancy between Dettori's reported explanation, and that provided to the clerk of the scales at Newbury before the late switch was announced.

"The clerk of the scales, and subsequently the stewards, were told an entirely different reason for the jockey change than the one given to the media," said Paul Struthers, the BHA's director of communications. "We are therefore looking into that as the tariff is not a satisfactory reason for a jockey change."

The same afternoon, Mick Channon withdrew two horses from an event for which only five had initially been declared, again as an apparent protest against a sub-tariff prize fund. However, the "self-certification" procedure for non-runners does not compel trainers to give a reason for a horse's withdrawal, and Channon will face no questions. The situation could change, however, if non-runners for tariff reasons become a regular occurrence.

Struthers said: "With regards to non-runners due to the tariff, as the rules are currently worded any reason given for self-certified non-runners is permitted, which is unfortunate as we do not believe that a race not meeting the tariff is an acceptable reason for choosing not to run once a horse had been declared, so we will be reviewing our options."